At E-commerce sites, a user executes a request sequence called session. Access to the E-commerce sites is made not only by a user who executes purchase processing but also by a user who executes browsing of new products, check of best-sellers, price comparison and the like. According to a request, a wide variety of processing is executed such as product search, cart processing, user registration•log-in, a shipping procedure and confirmation of purchase. Session here may be called as transaction in some cases.
E-commerce sites have a tendency of access avalanche in a specific time zone such as a weekend or a holiday. When rushing accesses exceed an allowance of a system of the site, there occurs a situation that responsiveness of the site will be sharply degraded or that the site becomes inaccessible. Sites incapability of handling an increasing number of user accesses satisfactorily results in losing a business chance.
As a method of preventing such situations as described above, for example, non-Patent Literature 1 discloses two methods shown in the following.
(a) To make a system structure have tolerance for an increase in the number of access.
(b) To dynamically change a structure of the system when the number of accesses is increased.
The method (a) has a problem that when access avalanche is temporary, most of servers forming the system will become idle servers, so that it is not a method effective in investment.
The method (b) is a method attaining effective use of servers by borrowing and lending servers with each other under an environment where a server is shared by other system when accesses avalanche to any of the systems. When the system is not designed to allow structure change with ease or when structure change requires a long period of time, however, it is difficult to cope with access avalanche by (b).
In a case where excessive investment to the system is impossible or where system structure change is not easy as described above, often adopted is a method of controlling a session. Although by limiting the number of sessions acceptable at the same time to a fixed number, system down is avoided to enable responsiveness to be maintained, a part of sessions should be rejected.
Examples of such a session control system are recited in Patent Literature 1 and non-Patent Literature 2.
The session control system in Patent Literature 1 is formed of a data base for storing an ID and a rank of a user, an information provision request accepting unit for accepting a provision request from a user's terminal through a network, a user specifying unit for referring to a rank corresponding to a user's ID included in an information provision request from the data base, a load monitoring unit for monitoring a web site load condition, and an information generating unit for generating information to be provided to a user according a rank of the user and a system load condition.
The information generating unit obtains a rank corresponding to an ID of a user from the data base, generates information to be provided to the user according to the rank and a load condition of a web site and transmits the information to a terminal. The rank is determined by a user's record of commercial transaction or the like.
Session control recited in Patent Literature 1 enables commercial transaction by a user having a strong possibility of producing profits to be continued by rejecting a session of a user whose rank is low when a system load is heavy. When ranking of a user is impossible or when there exist many users of the same rank, however, it is difficult to execute such session control as enables a web site load to be suppressed and profits to be increased.
The session control system disclosed in Non-Patent Literature 2 is formed of a session control unit for accepting or rejecting a newly connected session, a stage classification unit for classifying sessions into stages each indicative of a state of a session, a queue in which a classified session waits for its execution and a scheduling unit for taking out a session from the queue and executing the same on the web site.
The scheduling unit schedules a session waiting in the queue to make a profit of the web site be the highest. Rejection of a session by the system is executed only when a new session is set up. When priority of a session whose connection is allowed lowers, a response time becomes long, so that a session of a user who cannot wait longer is cut off. The longer a user waits for a response, the more dissatisfied he/she becomes.
On the other hand, in the system recited in Non-Patent Literature 2, scheduling of a request of a session belonging to the same stage is the same. Therefore, it is impossible to control a session continuing requesting without shifting a stage. More specifically, it will be difficult to reject such a session as continues browsing products without putting a product into a shopping cart.    Patent Literature 1: Japanese Patent Laying-Open No. 2006-065563.    Non-Patent Literature 1: Shinya Miyakawa, Nobuyuki Saji, Yutaka Kudo and Hideaki Tazaki, Business Grid Computing: 2. Technology Focus of Business Grid Middleware, Journal of Information Processing Society of Japan, Vol. 47, No. 9, p. 953-p. 961, September 2006.    Non-Patent Literature 2: Jakob Carlstrom, Raphael Rom, Application-Aware Admission Control and Scheduling in Web Servers, INFOCOM 2002.
First problem is that when accesses avalanche to a web site, the number of arrivals at processing whose importance is high such as purchase processing hits the top.
The reason is that when a web site is incapable of executing such discrimination of users as ranking, session control on a rank basis is impossible.
Another reason is a lack of an index for rejecting a session which fails to reach such processing as purchase processing whose importance is high, so called a stagnant session.
Second problem is that when accesses avalanche to a web site, user's dissatisfaction is increased.
The reason is that a session of a user whose priority is low with a weak desire for processing of high importance at a web site is not immediately rejected and continued after a long response time.